Of the aromatic compounds used in industry, benzene and xylenes are of outstanding importance on a volume basis. Those compounds are derived primarily from such aromatic naphthas as petroleum reformates and pyrolysis gasolines. The former result from processing petroleum naphthas over a catalyst such as platinum on alumina at temperatures which favor dehydrogenation of naphthenes. Pyrolysis gasolines are liquid products resulting from mild hydrogenation (to convert diolefins to olefins without hydrogenation of aromatic rings) of the naphtha fraction from steam cracking of hydrocarbons to manufacture ethylene, propylene, etc.
Regardless of aromatic naphtha source, it is usual practice to extract the liquid hydrocarbon with a solvent highly selective for aromatics to obtain an aromatic mixture of the benzene and alkylated benzenes present in the aromatic naphtha. That aromatic extract may then be distilled to separate benzene, toluene and C.sub.8 aromatics from higher boiling compounds in the extract. The benzene and toluene are recovered in high purity but the C.sub.8 fraction, containing valuable para xylene, is a mixture of the three xylene isomers with ethyl benzene. Techniques are known for separating p-xylene by fractional crystallization with isomerization of the other two isomers for recycle in a loop to the p-xylene separation. That operation is hampered by the presence of ethyl benzene (EB). However, a widely used xylene isomerization technique, "Octafining" can be applied. Octafining by passing the C.sub.8 aromatics lean in p-xylene and mixed with hydrogen over platinum on silica-alumina not only isomerizes xylenes but also converts ethyl benzene, thus preventing build-up of EB in the separation-isomerization loop.
The manner of producing p-xylene by a loop including Octafining can be understood by consideration of a typical charge from reforming petroleum naphtha. The C.sub.8 aromatics in such mixtures and their properties are:
______________________________________ Density Freezing Boiling Lbs./U.S. Point.degree. F. Point.degree. F. Gal. ______________________________________ Ethyl benzene -139.0 277.1 7.26 P-xylene 55.9 281.0 7.21 M-xylene -54.2 282.4 7.23 O-xylene -13.3 292.0 7.37 ______________________________________
Principal sources are catalytically reformed naphthas and pyrolysis distillates. The C.sub.8 aromatic fractions from these sources vary quite widely in composition but will usually be in the range 10 to 32 wt. % ethyl benzene with the balance, xylenes, being divided approximately 50 wt. % meta, and 25 wt. % each of para and ortho.
Calculated thermodynamic equilibria for the C.sub.8 aromatic isomers at Octafining conditions are:
______________________________________ Temperature 850.degree. F. ______________________________________ Wt. % Ethyl benzene 8.5 Wt. % para xylene 22.0 Wt. % meta xylene 48.0 Wt. % ortho xylene 21.5 100.0 ______________________________________
An increase in temperature of 50.degree. F. will increase the equilibrium concentration of ethyl benzene by about 1 wt. %, ortho xylene is not changed and para and meta xylenes are both decreased by about 0.5 wt. %.
Individual isomer products may be separated from the naturally occurring mixtures by appropriate physical methods. Ethyl benzene may be separated by fractional distillation although this is a costly operation. Ortho xylene may be separated by fractional distillation and is so produced commercially. Para xylene is separated from the mixed isomers by fractional crystallization.
As commercial use of para and ortho xylene has increased there has been interest in isomerizing the other C.sub.8 aromatics toward an equilibrium mix and thus increasing yields of the desired xylenes.
Isomerization processes operate in conjunction with the product xylene or xylenes separation processes. A virgin C.sub.8 aromatics mixture is fed to such a processing combination in which the residual isomers emerging from the product separation steps are then charged to the isomerizer unit and the effluent isomerizate C.sub.8 aromatics are recycled to the product separation steps. The composition of isomerizer feed is then a function of the virgin C.sub.8 aromatic feed, the product separation unit performance, and the isomerizer performance.
A typical charge to the isomerizing reactor may contain 17 wt. % ethyl benzene, 65 wt. % m-xylene, 11 wt. % p-xylene and 7 wt. % o-xylene. The thermodynamic equilibrium varies slightly with temperature. The objective in the isomerization reactor is to bring the charge as near to theoretical equilibrium concentration as may be feasible consistent with reaction times which do not give extensive cracking and disproportionation.
In Octafining, ethyl benzene reacts through ethyl cyclohexane to dimethyl cyclohexanes which in turn equilibrate to xylenes. Competing reactions are disproportionation of ethyl benzene to benzene and diethyl benzene, hydrocracking of ethyl benzene to ethane and benzene and hydrocracking of the alkyl cyclohexanes.
The rate of ethyl benzene approach to equilibrium concentration in a C.sub.8 aromatic mixture is related to effective contact time. Hydrogen partial pressure has a very significant effect on ethyl benzene approach to equilibrium. Temperature change within the range of Octafining conditions (830.degree. to 900.degree. F.) has but a very small effect on ethyl benzene approach to equilibrium.
Concurrent loss of ethyl benzene to other molecular weight products relate to % approach to equilibrium. Products formed from ethyl benzene include C.sub.8.sup.+ naphthenes, benzene from cracking, benzene and C.sub.10 aromatics from disproportionation, and total loss to other than C.sub.8 molecular weight. C.sub.5 and lighter hydrocarbon by-products are also formed.
The three xylenes isomerize much more selectively than does ethyl benzene, but they do exhibit different rates of isomerization and hence, with different feed composition situations the rates of approach to equilibrium vary considerably.
Loss of xylenes to other molecular weight products varies with contact time. By-products include naphthenes, toluene, C.sub.9 aromatics and C.sub.5 and lighter hydrocracking products.
Ethyl benzene has been found responsible for a relatively rapid decline in catalyst activity and this effect is proportional to its concentration in a C.sub.8 aromatic feed mixture. It has been possible then to relate catalyst stability (or loss in activity) to feed composition (ethyl benzene content and hydrogen recycle ratio) so that for any C.sub.8 aromatic feed, desired xylene products can be made with a selected suitably long catalyst use cycle.
Because of its behavior in the loop manufacture of p-xylene, or other xylene isomer, ethyl benzene is undesirable in the feed but is tolerated because of the great expense of removal from mixed C.sub.8 aromatics. Streams substantially free of ethyl benzene are available from processes as transalkylation of aromatics having only methyl substituents. Thus toluene can be reacted with itself (the specific transalkylation reaction sometimes called "disproportionation") or toluene may be reacted with tri-methyl benzene in known manner.
The transalkylation reactions provide means for utilizing the higher boiling aromatics separated in preparing BTX from reformates. Thus toluene may be reacted with tri-methyl benzenes to produce xylenes. They are also useful in handling high boiling aromatics formed by side reactions in such processes as isomerization of xylenes.
Another technique for utilizing the C.sub.9.sup.+ aromatics is described in U.S. Pat. No. 3,945,913 (Brennan and Morrison) dated Mar. 23, 1976. It is there shown that the alkyl aromatics of nine or more carbon atoms will react over certain acid catalysts exemplified by acid zeolite ZSM-5 to remove alkyl side chains of two or more carbon atoms and equilibrate the remaining methyl benzenes to provide the "BTX" mixture of benzene, toluene and xylenes. As that reaction was further studied, it was found that solid acid catalysts generally will promote the reaction sequence, although such catalysts as silica-alumina, zeolite Y and the like are less attractive than is zeolite ZSM-5 for the purpose. That finding is described and claimed in copending application Ser. No. 774,304, filed Mar. 4, 1977, by Brennan and Morrison.
A recent development in vapor phase isomerization is described in U.S. Pat. No. 3,856,872, (Morrison) dated Dec. 24, 1974. It is there shown that use of a catalyst such as HZSM-5 in combination with a metal having hydrogenation/dehydrogenation promoting capability under essentially Octafining conditions is very efficient for isomerization of xylenes at reduced hydrogen flow as compared with Octafining. The extent of xylene loss is substantially reduced by this change of catalyst. Concurrently, the mechanism of ethyl benzene conversion is drastically changed on substitution of, e.g. NiHZSM-5, for the platinum on silica/alumina of Octafiners. The Morrison process results in conversion of ethyl benzene by transalkylation reactions including disproportionation of ethyl benzene to benzene and diethyl benzene, disproportionation and ethylation of xylene and the like producing alkyl aromatic compounds of nine or more carbon atoms (C.sub.9.sup.+) together with benzene and toluene. Those conversion products are readily separated in the loop for recovery of p-xylene and isomerization of o- and m-xylenes. In general, loss of xylenes increases as severity of the isomerizer is increased to enhance the conversion of ethyl benzene.
In Bonacci et al U.S. Pat. No. 3,957,621, dated May 18, 1976, are described various combinations of aromatic processing steps in combination of differing nature.